Ask Dr. Knowledge

Centipedes never have 100 legs, and their bite is venomous

August 30, 2010

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Centipedes creep me out. Are they dangerous? Do they really have 100 legs?

Centipedes have segmented bodies, typically with one pair of legs per segment. While their name literally means “100 legs,’’ adults actually have from 30 to more than 300 legs — but never precisely 100. That’s because adults always have an odd number of pairs of legs, so while they could have 15 or 101 pairs, they would never have 50. The American house centipede has an even number of pairs, four, when it hatches from an egg. But later, as it grows and moults, it has five, seven, nine, 11, and 15 pairs before it reaches maturity.

Each pair of legs is a little longer than the pair in front of it, so that they don’t overlap and to make it less likely they will trip over each other. The back ones can be twice as long as the front ones.

Now on to the issue of danger, which is actually linked to the discussion of legs. What would otherwise be the first pair of legs is a pair of venom-injecting claws called forcipules, which are unique to centipedes. They are hollow and connected to poison glands.

Small centipedes usually don’t have long enough forcipules to get through human skin, so despite the venom, they’re no threat. Larger centipedes can puncture skin with these claws, but in most cases all that happens is local irritation, quite similar to a bee sting, possibly accompanied by headache and nausea. Palpitations and anxiety can also occur. For people with severe allergies, the bite can induce a severe and possibly fatal reaction.

In Massachusetts, where centipedes are small, I wouldn’t worry much, but in general I wouldn’t advise playing with them in any way that would let them bite. There are more than 8,000 species of centipede worldwide. Larger varieties exist in the tropics and can be much more worrisome. Their bites can produce severe fever and cramps. The largest is the Amazon giant centipede, which can be over a foot long. Big centipedes can eat lizards, birds, frogs, mice, and bats, and you most definitely would not want to be bitten by one.

Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.